The invention relates to a glass-ceramic composition having a low helium permeability and especially suitable for sealing to molybdenum.
Some requirements that are generally recognized as necessary for envelope materials for such as gas tubes or for long-life vacuum tubes are that the materials have a low permeability to gases, particularly helium and hydrogen, and that the materials be sufficiently refractory to permit high-temperature brazing and bake-out of the tubes. Tube designers have attempted to cope with these requirements by employing solder coated nickel-iron-cobalt alloys or by employing molybdenum with alumina ceramics for insulator parts. Use of these materials has presented additional problems in that the expansion characteristics generally do not match those of the ceramics thereby resulting in restrictions on designs. Additionally, prior tube materials have necessitated that fired ceramic be machined to achieve the desired configurations and have also necessitated multiple firings to initially deposit the braze material on the ceramic and metal surfaces to be contacted and to subsequently seal these surfaces to a seal member and thereby make ceramic-to-metal seals, resulting in fabrication processes and products that are difficult to accomplish and not economically desirable.
In some instances, hydrogen and helium impermeability is desired because the gas or vacuum tubes may be operated in a gaseous environment, such as a hydrogen or helium environment, and permeation of these gases into the tubes would result in a premature failure or a detrimental effect upon the tube by adversely affecting the tube hold-off strengths, tube switching characteristics, or the like.
The development of glass-ceramics, that is, materials that are melted and formed as a glass and subsequently crystallized to form a ceramic, has done much to alleviate prior art problems. However, known glass-ceramics still have limitations, including an objectionable differential between the thermal expansion of the glass ceramic and the molybdenum, a relatively high permeability to helium, and a relatively high temperature requirement for sealing to molybdenum such as, for example, from about 1000.degree. to about 1050.degree.C.